The present invention relates to the field of displays, and, more particularly, to a rugged display for use in severe shock and vibration environments.
Liquid crystal displays (LCDs) are widely used to convey information to a user, especially where the information is generated by a computer or processor. For example, an LCD is commonly used in a personal computer, a portable data terminal, to display information to a user. An LCD may also find use in aircraft and other vehicles that may subject the LCD to severe shocks or vibration.
An LCD typically includes a pair of opposing glass cover plates with the liquid crystal material positioned between the cover plates. This structure is typically referred to as an LCD sandwich. One or more polarizing layers may be joined to the glass cover plates for a typical LCD. Unfortunately, the liquid crystal material may be readily damaged by shock or vibration imparted to the display.
Attempts have been made in the past to ruggedize a conventional LCD to survive shocks, such as from dropping and/or vibration. One or more supporting plates may be joined to the front or back surfaces of the LCD sandwich, as disclosed, for example, in U.S. Pat. No. 5,606,438 to Margalit et al. The patent further discloses a ruggedized LCD incorporating layers of adhesive which extend continuously across the front and back surfaces of the LCD sandwich. The adhesive layers join the LCD sandwich to a front glass plate and a rear diffuser.
The adhesive layer on the front is described as increasing the moment of inertia of the LCD sandwich by causing the LCD sandwich and front glass to behave as single unit. Accordingly, localized stress is reduced when the unit as a whole suddenly decelerates in a drop test, for example. In addition, the thickness of the adhesive layer is disclosed as between 4 to 20 or 30 microns to thereby allow differential thermal expansion between the LCD sandwich and the front glass, for example. Further, the front glass plate includes edge portions which extend outwardly and are captured in a corresponding recess of a plastic mounting frame so that the LCD is suspended from the front glass plate carried by the frame.
U.S. Pat. No. 5,150,231 to Iwamoto et al. discloses another approach to ruggedizing an LCD. In particular, the patent discloses an LCD panel which is mounted to a frame by elastic members. In addition, the frame defines an almost enclosed space behind the LCD which traps air to dampen motion and thereby further protect the LCD from dropping or vibration.
Unfortunately, conventional approaches to ruggedizing an LCD may not be successful in protecting the relatively delicate liquid crystal material against shock and vibration. This may be so especially in a vehicle, such as an aircraft, which may have severe and sustained vibration levels.
Conventional attempts to ruggedize an LCD have also overlooked the compatibility between frame materials and the glass, for example, of the LCD. A rigid plastic frame, for example, has a different coefficient of thermal expansion compared to glass and may cause the LCD glass to break under extremes of temperature. Accordingly, operation over relatively wide temperature ranges may require complicated mounting techniques or the effective operating temperature may be restricted. In addition, attempting to isolate the glass from the frame may increase difficulties with shock and vibration.
In view of the foregoing background, it is therefore an object of the present invention to provide an LCD which is resistant to high shock and vibration levels.
It is another object of the present invention to provide an LCD which is readily mounted while taking into account compatibility of the frame material and the LCD materials.
These and other objects, advantages and features of the present invention are provided by an LCD including a first panel assembly and a second panel assembly with a liquid crystal material layer positioned therebetween, and wherein the first panel assembly has a resonant frequency substantially the same as a resonant frequency of the second panel assembly so that the LCD is resistant to damage from shock and vibration. The first panel assembly preferably comprises a first cover panel immediately adjacent the liquid crystal material layer and at least one additional panel positioned adjacent the first cover panel. Similarly, the second panel assembly may comprise a second cover panel immediately adjacent the layer of liquid crystal material and at least one additional panel adjacent the second cover panel. In other words, a typical LCD sandwich including the first and second cover panels with the liquid crystal material layer therebetween, further includes one or more additional support panels.
Considered in different terms, the first panel assembly has substantially matched mechanical properties to the second panel assembly. For example, the substantially matched mechanical properties preferably include a stiffness to mass ratio. In addition, the resonant frequency is preferably a first mode resonant frequency. Accordingly, damage caused by the relative motion between the first and second assemblies is avoided by matching the first and second assemblies so that they may move together.
For additional resistance to shocks, each of the assemblies may include a support panel, such as provided by an additional glass plate. A front assembly, may also include at least one filter panel or plate.
Another aspect of the invention relates to a frame mounted around a periphery of the first and second panel assemblies. Preferably, the first panel assembly has a coefficient of thermal expansion substantially the same as a coefficient of thermal expansion of the second panel assembly. For additional compatibility, the frame also preferably has a coefficient of thermal expansion substantially the same as the coefficient of thermal expansion of the first and second panel assemblies.
A method aspect of the invention is for making a liquid crystal display (LCD) resistant to damage from vibration. The liquid crystal display is preferably of a type including a liquid crystal material layer positioned between two cover panels. The method preferably comprises the step of positioning at least one additional panel adjacent at least one cover panel to define first and second panel assemblies on opposite sides of the liquid crystal material layer so that the first panel assembly has a resonant frequency substantially the same as a resonant frequency of the second panel assembly.